From 07760822db8dab9dc2bec7d2af23d804ab19da22 Mon Sep 17 00:00:00 2001
From: Josh Triplett <josh@joshtriplett.org>
Date: Thu, 26 Jun 2025 15:06:31 -0700
Subject: [PATCH] mbe: Fold calls to `check_meta_variables` into the parser
 loop

---
 compiler/rustc_expand/src/mbe/macro_check.rs | 23 ++++++--------------
 compiler/rustc_expand/src/mbe/macro_rules.rs |  3 +--
 2 files changed, 8 insertions(+), 18 deletions(-)

diff --git a/compiler/rustc_expand/src/mbe/macro_check.rs b/compiler/rustc_expand/src/mbe/macro_check.rs
index dc2d46c4a141..bbdff866feba 100644
--- a/compiler/rustc_expand/src/mbe/macro_check.rs
+++ b/compiler/rustc_expand/src/mbe/macro_check.rs
@@ -105,8 +105,6 @@
 //! stored when entering a macro definition starting from the state in which the meta-variable is
 //! bound.
 
-use std::iter;
-
 use rustc_ast::token::{Delimiter, IdentIsRaw, Token, TokenKind};
 use rustc_ast::{DUMMY_NODE_ID, NodeId};
 use rustc_data_structures::fx::FxHashMap;
@@ -190,29 +188,22 @@ struct MacroState<'a> {
     ops: SmallVec<[KleeneToken; 1]>,
 }
 
-/// Checks that meta-variables are used correctly in a macro definition.
+/// Checks that meta-variables are used correctly in one rule of a macro definition.
 ///
 /// Arguments:
 /// - `psess` is used to emit diagnostics and lints
 /// - `node_id` is used to emit lints
-/// - `span` is used when no spans are available
-/// - `lhses` and `rhses` should have the same length and represent the macro definition
+/// - `lhs` and `rhs` represent the rule
 pub(super) fn check_meta_variables(
     psess: &ParseSess,
     node_id: NodeId,
-    span: Span,
-    lhses: &[TokenTree],
-    rhses: &[TokenTree],
+    lhs: &TokenTree,
+    rhs: &TokenTree,
 ) -> Result<(), ErrorGuaranteed> {
-    if lhses.len() != rhses.len() {
-        psess.dcx().span_bug(span, "length mismatch between LHSes and RHSes")
-    }
     let mut guar = None;
-    for (lhs, rhs) in iter::zip(lhses, rhses) {
-        let mut binders = Binders::default();
-        check_binders(psess, node_id, lhs, &Stack::Empty, &mut binders, &Stack::Empty, &mut guar);
-        check_occurrences(psess, node_id, rhs, &Stack::Empty, &binders, &Stack::Empty, &mut guar);
-    }
+    let mut binders = Binders::default();
+    check_binders(psess, node_id, lhs, &Stack::Empty, &mut binders, &Stack::Empty, &mut guar);
+    check_occurrences(psess, node_id, rhs, &Stack::Empty, &binders, &Stack::Empty, &mut guar);
     guar.map_or(Ok(()), Err)
 }
 
diff --git a/compiler/rustc_expand/src/mbe/macro_rules.rs b/compiler/rustc_expand/src/mbe/macro_rules.rs
index 234e0257530c..dad2fd99ef20 100644
--- a/compiler/rustc_expand/src/mbe/macro_rules.rs
+++ b/compiler/rustc_expand/src/mbe/macro_rules.rs
@@ -423,6 +423,7 @@ pub fn compile_declarative_macro(
         .pop()
         .unwrap();
         check_emission(check_rhs(sess, &rhs_tt));
+        check_emission(macro_check::check_meta_variables(&sess.psess, node_id, &lhs_tt, &rhs_tt));
         lhses.push(lhs_tt);
         rhses.push(rhs_tt);
         if p.token == token::Eof {
@@ -438,8 +439,6 @@ pub fn compile_declarative_macro(
         return dummy_syn_ext(guar);
     }
 
-    check_emission(macro_check::check_meta_variables(&sess.psess, node_id, span, &lhses, &rhses));
-
     let transparency = find_attr!(attrs, AttributeKind::MacroTransparency(x) => *x)
         .unwrap_or(Transparency::fallback(macro_rules));